This invention relates to improvements in sound reproduction and particularly to a novel and improved ambient expansion loudspeaker system for providing improved ambient imaging without regard to the listener position relative to the loudspeaker system.
Generally speaking, loudspeaker systems comprising one or more drivers (sometimes referred to as speakers) are arranged in an enclosure or housing in such a way as to reproduce the full audio frequency spectrum. The individual drivers or speakers may also be arranged so as to radiate the audio spectrum or various portions thereof, in some preselected pattern.
This application utilizes the terms "loudspeaker" or "loudspeaker system" to refer to an apparatus having an enclosure in which one or more drivers are mounted and electrically connected for radiating audio signals. The term "driver" will be used herein to refer to the individual drivers or speakers which may be mounted in such a loudspeaker system. Such drivers commonly include at least one radiating element such as a cone, horn, dome or the like and an electrically driven element or elements such as one or more voice coils or windings to which electrical signals are applied. These voice coils or windings convert the electrical signals into mechanical motion of the cone or other movable sound radiating element to produce corresponding acoustical signals, that is, sound vibrations.
In stereophonic sound recordings or broadcasts, the dimensional content of the signal is generally dependent upon the location and spacing of one or more microphones. For example, if a right channel microphone and a left channel microphone are used in the recording or broadcast, separate left and right loudspeaker systems may be utilized to produce a program with dimensional content similar to the spatial or ambient characteristics in the original location of the recording or broadcast. In this regard, phase and amplitude differences between what is recorded or reproduced in the left channel versus the right channel can cause the ear/brain mechanism to interpret the resulting sound reproduction as having a spatial content or reality. However, for the ear/brain mechanism to be convinced of the spatial effect, it is necessary that the left and right channel sonic information reach the listener's left and right ears independently and in a time sequence consistent with the "ambient signature" of the original recording or broadcast.
The acoustic credibility of loudspeaker reproduction is compromised in many prior art loudspeaker designs by the commingling of left and right channel spatial information in both of the listener's ears. The cranial "shield" between the left and right ears does not adequately isolate or shield the left and right ears from each other such that right channel information will be perceived in the left ear, and vice versa, left channel information will be perceived in the right ear. This interaural mixing of left and right channel information compromises the ear/brain mechanism's full employment of its differential ability to perceive and localize.
In addition to the foregoing, the interaction of the loudspeakers with the listening room creates dimensional compression in the lateral plane which further diminishes the spatial credibility of the listening experience. By "dimensional compression" is meant that acoustic events occurring at right angles to a listener situated in a concert hall are often perceived through loudspeakers as though they were squeezed between the left and right loudspeaker systems. Listeners often attempt to regain some of the dimensional information by spacing the left and right loudspeakers systems as far apart as possible in the listening room. However, when this is done the central image and its specificity are lost, such that the individual performers seem to be split between the left and right loudspeakers.
The two principal elements in lateral localization of sound are time (phase) and intensity. Thus, a louder sound seems closer and sound arriving later in time seems farther away. The listener employs the two ears and the perceptive interval between the two ears to establish lateral location. This involves the so-called Pinnar effect, often discussed in terms of "interaural cross-correlation". In the typical prior art loudspeaker system arrangement, the listener is positioned in front of and equidistant from a pair of loudspeaker systems which are adjusted to radiate approximately equal volumes.
Whatever dimensional quality is gained by the listener is created when the listener is able to compare the acoustic events through the ear/brain mechanism. An imaginary "difference component" is perceived between the left and right loudspeaker in the area between the two loudspeakers, and this difference component diminishes as the listener moves from the equidistant position as a function of the inverse square loss of intensity with proximity. As mentioned above, the independent right ear and left ear perception are also compromised by some left ear perception of the right channel and vice versa. Since the stereophonic or spatial effect of the recorded or broadcast material depends on the listener's perception of the difference between the left and right channels, any reduction in either of the channels due to movement of the listener away from the equidistant position tends to further compromise this situation.
In this regard, some listeners have expressed a preference for stereophonic headphones. Headphones isolate the recorded, time-related ambience by blocking out the impinging ambience of the listening room. They also prevent the arrival of left information at the right ear and vice versa. However, headphones provide limited acoustic performance, principally in the bass region, and are also confining in terms of the listener's freedom of movement, as well as being somewhat uncomfortable to wear for long periods of time. Moreover, the perceived "soundstage" moves with any movement of the wearer's head, rather than remaining spatially fixed, as it ought to for a realistic spatial effect as the listener turns his head.
FIG. 1 illustrates a hypothetical solution to the problem (albeit highly impractical) involving loudspeakers and a wall-barrier which separates the listener's left and right ears in a manner which in effect extends the acoustical "shadow" of the human cranium that separates the left and right ears. This "cranial shadow" alone is effective enough in isolating left and right difference information for wavelengths smaller than the space between left and right ears, for which intensity differences provide localization. However, as wavelengths become longer below roughly 1000 hz, the ear-brain mechanism's sensitivity to phase differences becomes the operative element for providing localization.
These longer wavelength phase differences provide almost all of the dimensional information about the physical size of the original recording hall or room. In two-channel stereo, with the loudspeakers in front of a listener, the interaural commingling referred to is perceived as a narrowing of the frontal "soundstage."
Polk, U.S. Pat. No. 4,489,432 states that an acoustic event occurring at right angles to the primary acoustic event such as the first reflection from the side walls of the concert hall or recording studio, is perceived in conventional prior art loudspeaker systems as though it came from the space between the left and right loudspeakers. Polk further postulates a two-microphone recording where the microphone spacing involved exceeds the cranial separation of the left and right ear.
Some years ago designers learned that a synthesized "difference" component could be extracted from two-channel stereophonic material by simply inverting the relative polarity of the left and right channels so as to obtain either "plus left minus right" (+L-R) or "plus right minus left" (+R-L). In this manner the monophonic component is disabled. Any left or right signal of identical intensity and phase is completely cancelled, leaving only the difference between channels. Conversely, adding the two channels in phase produces Left plus Right (L+R) or pure monophony since any difference signal is combined as a single component.
Researchers learned that the extracted differential L-R or R-L could be independently reproduced through its own dedicated transducers to achieve a variety of effects. When isolated, the differential L-R, R-L was found to simulate the reflection and phase characteristics of the recording studio or concert hall occurring chiefly in the later arrival time domain. When combined with the primary left and right early arrival information a very credible acoustic experience of spatiality could be simulated.
In motion picture applications the differential L-R, R-L component was encoded (often with 3-20 millisecond time delay) and then decoded and reproduced in transducers located at the rear and side walls of the theater. In such a complex acoustic situation a "center-channel" L-R monophonic transducer is often employed to provide the necessary on-screen dialogue specificity and synchronization.
Numerous methods of extracting the differential L-R, R-L information exist. The inventor herein is also the inventor in two prior U.S. Pat. Nos. 4,847,904 and 5,117,459. These patents disclose multiple driver speaker systems employing at least one dual-coil dedicated driver for each of the left and right loudspeaker systems, connected in opposing polarity to produce L-R and R-L differential information. These dedicated differential drivers are mounted adjacent to and angled outwardly from the associated "left" and "right" drivers.
Still another method of extracting differential L-R, R-L information is disclosed in Polk U.S. Pat. Nos. 4,497,064 and 4,489,432; Hafler 3,697,692. The method disclosed in these patents specifies single coil drivers connected from positive Left and Right amplifier terminals.
Polk U.S. Pat. No. 4,489,432 discloses left and right "outboard" loudspeaker systems equipped with left and right subspeakers connected electrically so as to obtain L-R and R-L signals, in addition to L+ and R+ "inboard" drivers. This system solves for the geometric triangulation of sound paths for a listener located equidistant from each left and right speaker system in such a way that the destructive co-mingling of left and right information in the left and right ears is cancelled. However, the listener must remain equidistant to all times to achieve this effect. In order to create a perceptible time delay between the left and right primary drivers and the L-R and R-L subspeaker drivers, the "inboard" and "outboard" drivers are separated by a finite lateral spacing corresponding to roughly the left-right ear spacing. All of the drivers radiate sound along parallel axes. In this way a precise geometric triangulation situated equidistant between the Left and Right speaker systems. This triangulation is so critical that even small head movements can create image shifts.
Arnston discloses in U.S. Pat. No. 4,586,192 a system that purports to expand the acoustic image by providing an anti-phase signal extracted from the opposite channel. In other words the left speaker receives a right signal in opposite phase, which is fed to a secondary "enhancement" winding of a dual voice coil, and vice versa for the right speaker. This arrangement is such that the right speaker reproduces R-L while the left speaker reproduces L-R. Very significant attenuation of bass frequencies would result were it not for an elaborate filter network which sharply reduces the bass signal in the "enhancement" voice coil winding of each speaker.